Nitrogen-rich mesoporous carbon materials were obtained by pyrolyzing gelatin between 700 and 900 C with a nano-CaCO 3 template. The mesoporous structure and the high nitrogen content endowed these materials with reversible capacities up to ca. 1200 mA h g À1 . The high specific surface area and the nitrogen doping are responsible for the capacity loss in the initial cycle. FTIR and XPS studies indicate that the nitrogen in the material exists in the form of pyridinic, pyrrolic/pyridonic and graphitic nitrogen. The Raman spectroscopic analysis indicates that the structure of the mesoporous carbon becomes more disordered during discharge and is restored during recharge, a behavior similar to that in nitrogen-free hard carbon materials. The reversible structural variation of these carbon materials ensures their high cyclic reversibility.

Layered Na-based oxides with the general composition of Na x TMO 2 (TM: transition metal) have attracted significant attention for their high compositional diversity that provides tunable electrochemical performance for electrodes in sodium-ion batteries. The various compositions bring forward complex structural chemistry that is decisive for the layered stacking structure, Na-ion conductivity, and the redox activity, potentially promising new avenues in functional material properties. In this work, we have explored the maximum Na content in P2-type layered oxides and discovered that the high-content Na in the host enhances the structural stability; moreover, it promotes the oxidation of low-valent cations to their high oxidation states (in this case Ni 2+ ). This can be rationalized by the increased hybridization of the O(2p)-TM(3d-e g *) states, affecting both the local TM environment as well as the interactions between the NaO 2 and TMO 2 layers. These properties are highly beneficial for the Na storage capabilities as required for cathode materials in sodium-ion batteries. It leads to excellent Na-ion mobility, a large storage capacity (>100 mAh g −1 between 2.0-4.0 V), yet preventing the detrimental sliding of the TMO 2 layers (P2−O2 structural transition), as reflected by the ultralong cycle life (3000 (dis)charge cycles demonstrated). These findings expand the horizons of high Na-content P2-type materials, providing new insights of the electronic and structural chemistry for advanced cathode materials.

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